1. Field of the Invention
The present invention relates to transducers and to tools utilizing such transducers. In another aspect, the present invention relates to acoustic transducers for use in logging-while-drilling tools ("LWD") having such transducers. In even another aspect, the present invention relates to acoustic transducers having integral pressure stabilization, and to tools utilizing such transducers.
2. Description of the Related Art
In general, the apparatus for making an acoustic log consists of a transmitter capable of providing acoustical impulses, and at least one receiver responsive to acoustical energy, both mounted on a support for movement through the length of the well bore. The transmitter and receiver are spaced apart by a fixed distance and as the support is moved through the well bore, the energy from acoustic pulses periodically generated by the transmitter is picked up at the receiver after transmission through the well bore. The amplitudes of the received signals are correlated with the depth in the well bore to provide a log indicating the qualities of the cement bonding to the bore over the length of the well.
The acoustic transmitter and receivers employed for these purposes usually comprise a piezoelectric or magnetostrictive element which, in the case of a transmitter, is actuated by an electrical impulse to generate an acoustic wave and which, in the case of the receiver, is responsive to acoustical energy impinging theron to generate an electrical impulse to generate an acoustic wave and which, in the case of the receiver, is responsive to acoustical energy impinging thereon to generate an electrical signal generally proportional to the intensity of the acoustic energy.
For effective operation of these components, it is necessary that they be both efficiently coupled to the surrounding media and at the same time carefully protected from the deleterious effects of the liquids and gases present in well bores or casing.
These requirements present problems, especially in view of the temperature and pressure variation encountered as the apparatus moved through different levels in the well bore. The apparatus must not only be impervious to the well pressures, but must also avoid affecting the acoustic properties thereof.
Flexible elastomer covers have been utiized to enclose transducer elements in an oil. While these covers were liquid tight, they did not prevent gases in the borehole fluid from permeating under the high pressures encountered and having, the gases go into solution in the oil. Hence, when the logging tool was withdrawn from the wellbore, the gases could not leave at a rate adequate to prevent bursting of the cover. Moreover, earlier types of logging tools were difficult to repair by virtue of the techniques employed to seal them against well fluids and to equalize them against well fluids and to equalize interior and exterior pressures.
U.S. Pat. No. 3,213,415, issued Oct. 19, 1965 to Moser et al., discloses a pressure equalizing arrangement for acoustic logging, in which the well tool includes a central supporting mandrel around which the transducer element is mounted to provide an annular cavity therebetween. The cavity is completely filled with an electrically non-conductive liquid such as silicone oil which is also relatively compressible. For effective operation of the transducer element, it is necessary that the pressure in the transducer cavity be equalized with the pressure of the wellbore fluids. Bellows means are provided in communication with both the wellbore fluids and the oil-filled cavity, such that the external pressures are communicated to the oil in the cavity, thereby maintaining both the internal and external pressures substantially equal.
This type of pressure equalizing arrangement of utilizing an oil filled cavity in the tool into which each of the transducers are positioned is still in use today.
One of the major drawbacks of such a system is the maintenance problems. Specifically, removal of any of the transducers, for example for servicing or repair, causes some of the oil to leak from the system, and as such, draws air into the oil system. As a hydraulic system, this oil must then be "bled" from the system, and oil refilled. The operation of bleeding air from the oil system and refilling, will easily consume three, four or more hours.
Thus, there is a need in the art for an improved system for equalizing pressure between the wellbore fluids and the transducer cavity.
These and other needs in the art will become apparent to those of skill in the art upon review of this patent specification, including its drawings and claims.